Seamless tube is usually manufactured by the Mannesmann plug mill process or the Mannesmann mandrel mill process. In order to manufacture seamless tube by such a process, first, a solid rod-shaped billet (referred to in this description simply as a billet) is introduced into a heating furnace and heated therein to a predetermined temperature. The billet is then removed from the heating furnace and is rolled for piercing in a piercer to produce a hollow shell. The hollow shell is then rolled for elongation using a plug mill or a mandrel mill or a similar rolling mill in which primarily the wall thickness of the hollow shell is reduced. Thereafter, it is rolled for sizing using a reducing mill such as a sizer or a stretch reducer in which primarily the outer diameter thereof is reduced to manufacture a seamless tube having desired dimensions.
In Patent Document 1, the present inventors disclosed an invention in which a billet is pierced using a piercer comprising skew rolls and grooved disk rolls each having an optimized roll shape, thereby making it possible to perform piercing with high efficiency without the occurrence of miss-rolling (a state in which the advance of the material being rolled stops) while suppressing an increase in the outer diameter of the bottom portion of the resulting hollow shell under such conditions that the expansion ratio Exp (outer diameter of the hollow shell/outer diameter of the billet) is at least 1.15.
In Patent Document 2, the present inventors also disclosed an invention in which piercing of a billet is performed while controlling the rotary forging effect and preventing the occurrence of internal surface flaws by optimizing the ratio of the rotational frequency (rotating speed) of a billet in the steady state region up to the tip of a plug (as will be explained while referring to the graph in FIG. 1, this is the region from LE2 onwards in which the speed of advance of the billet becomes roughly constant after the start of piercing) to the rolling reduction of the outer diameter of the billet depending on the ratio between the diameter of the skew rolls of a piercer at its inlet and the diameter of the gorge portion of the skew rolls, the rotational frequency of billet being determined by the predetermined roll inclination angle, the piercing ratio, and the piercing efficiency.
Patent Document 1: JP 3021664 B2
Patent Document 2: WO 2004/103593